These primordial rocks could hardly be more different. Siwa will be the largest asteroid ever encountered by a spacecraft, while (apart from a tiny asteroid moon called Dactyl) Otawara will be the smallest. Otawara is suspected to be a chunk of once-molten basalt (a type V or SV asteroid), and Siwa seems to be a carbon-rich object, which is blacker than coal (a type C asteroid).
 
 

Asteroid Vital Statistics 4979 Otawara 140 Siwa Average distance from Sun (million km) 324 409 Orbital period (years) 3.19 years 4.52 years Size (estimated) 2.6 - 4 km 110 km Rotation period (estimated) 2 h 42 4 h 38 m Orbital inclination (degrees) 0.91 3.19 Orbital eccentricity 0.1449 0.2157 Asteroid type V or SV C Date of discovery 2 Aug. 1949 13 Oct. 1874 Discoverer K. Reinmuth J. Palisa

 
Rosetta's flyby of Otawara will take place on 12 July 2006, when the asteroid is 283 million km from the Sun. Travelling at a relative velocity of more than 10 km/sec, the spacecraft will pass by Otawara's sunlit side at a distance of about 2200 km. Otawara rotates faster than any asteroid so far visited by spacecraft (about once every 3 hours), which should allow Rosetta to image most of its surface during the flyby.

Rosetta will also obtain spectacular images as it flies to within 3500 km of Siwa on 24 July 2008.The spacecraft will fly past at a relative velocity of 17 km/sec, approaching on the sunlit side and then looking at a crescent phase as it moves away. At this time, Siwa will be about 470 million km from the Earth, so that signals from the spacecraft will take 26 minutes to reach ground stations.